I had the same pull out faucet in my kitchen for more than 6 years and decided to go with a pull out gooseneck. I started with an American Standard faucet from Home Depot. Everything went fine. 3 weeks later, we started getting a smell and bad taste in the water whenever the faucet had not been used for a while. The smell and taste would go away after running the water for 30-60 seconds. I was able to eliminate the drain as the problem, checked all the screens and found no evidence of anything in the pull out head. I assumed it was the faucet and went back and got a Kohler gooseneck. After the change over there was no bad smell or taste. 3 weeks later I have the same problem and do the same things over again. Now I abandon the Gooseneck and get a Kohler almost identical to the faucet I had for years. Again, no smell or taste after the change. 3 weeks later same smell and taste back again. There is no smell in any other water source in the house, I have well water with no treatment systems, and the water was tested and had no issues less than a year ago. I've called Kohler and they do not know of a similar problem. I am waiting for new supply lines and pullout hose from them to change one at a time to see where the smell may be coming from. In the mean time, has anyone ever heard of or had a similar problem?

I'd say that it is the supply lines. There may be something in your water that is reacting with the material the supply lines are made of.

That gave me an idea, I got under the sink and took the pull out hose out of play and had my wife turn the water on. There was no smell from the water from the copper line feeding the pullout hose. When I reconnected the pullout hose the smell was there. It has to be in the pull out hose. But, what can I do? Just keep trying different brands until I find one that doesn't stink?

YEP

or

contact the manufacturer

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

So, I contacted Kohler. They sent me new supply lines and a new pull out hose. I changed the pull out hose first, smell was gone. Never changed the supply lines. Two weeks later the smell was back. Called Kohler, they said it is something in the water reacting with their hose and their fix was return it and get something with a side sprayer. Before I resort to that, does anyone know of a brand/manufacturer that may use a different type of hose material than Kohler and American Standard for their pull out hoses?

I’m having the same exact problem with my Kohler pull out faucet as you. They sent me a new hose that I’ll install but I suspect I’ll have the same issue. Did you find a faucet that works for you?